Creep behaviour of Ti-6Al-2Sn-4Zr-2Mo: II. Mechanisms of deformation

Constant load creep tests are performed in Ti-6242(Si) alloy with a lath microstructure, at temperatures of 538 and 565 °C. A change in the stress exponent values from ˜1 at low stresses to between 5 and 7 at high stresses, is indicative of a change in creep mechanism. TEM analysis indicates that the deformation is dominated by a-type dislocations in the phase, with little evidence of dislocation activity in the β laths. At higher stress (310 MPa), the a-type dislocations are pinned frequently along their screw direction by tall jogs. A creep model is proposed based on the premise that movement of these jogged screw dislocations may control the creep rate. In contrast, at low stress (172 MPa), the a-type dislocations have long straight screw segments with no apparent pinning points. The near-edge segments are in climb configurations. The creep rates here are close to those predicted, based on Harper–Dorn creep, although the dislocation density is larger than that normally associated with this regime.     

Pinning of dislocations and the origin of the stress anomaly in FeAl alloys

The anomalous stress rise found at intermediate temperatures in FeAl alloys may be caused by the presence of thermal vacancies produced as the temperature rises. Strong support for this hypothesis is provided by the demonstration that the same absolute values of stresses as well as stress increases are found both when testing at high temperatures and when testing at room temperature samples quenched from the same high temperatures. Examination of the superdislocations present after deformation shows strong pinning only at jogs produced by intersection with forest dislocations. Such sessile jogs on screw superdislocations lead to dipole and loop formation as the dislocations continue moving, hence producing the debris observed after deformation. In addition, edge superdislocations show a stepped morphology caused by line instabilities over a certain range of directions. There is no evidence of strong pinning and associated dislocation bowing by vacancy-aggregate type obstacles, and it is, therefore, deduced that the pinning obstacles responsible for the anomalous stress increase are probably relatively weak single vacancies, rather like solute atoms, and not stronger multi-vacancy defects.     

Spacing defects and disconnections in grain boundaries

The process of cutting of a grain boundary by a gliding or climbing dislocation is considered. Some planar dislocation arrays with long-range stress fields are also treated. Defects formed on the grain boundaries by these mechanisms include edge and screw disconnections, grain boundary dislocations, spacing defects and line forces. The cutting defects can also acquire kinks and jogs. The results have implications for emission of lattice dislocations from grain boundaries, trapping of dislocations at grain boundaries and grain boundary topography.     

Creep behaviour of a cast TiAl-based alloy for industrial applications

The creep behaviour of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) was investigated. Constant load tensile creep tests were performed in the temperature range 973–1073 K and at applied stresses ranging from 200 to 390 MPa. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent n is determined to be 7.3 and true activation energy for creep Q is calculated to be 405 kJ/mol. The initial microstructure of the alloy is unstable during creep exposure. The transformation of the α₂(Ti₃Al)-phase to the γ(TiAl)-phase, needle-like B2 particles and fine Ti₅Si₃ precipitates and particle coarsening are observed. Ordinary dislocations in the γ-matrix dominate the deformation microstructures at creep strains lower than 1.5%. The dislocations are elongated in the screw orientation and form local cusps, which are frequently associated with the jogs on the screw segments of dislocations. Fine B2 and Ti₅Si₃ precipitates act as effective obstacles to dislocation motion. The kinetics of the creep deformation within the studied temperature range and applied stresses is proposed to be controlled by non-conservative motion of dislocations.     

A molecular dynamics study of self-diffusion in the cores of screw and edge dislocations in aluminum

Self-diffusion along screw and edge dislocations in Al is studied by molecular dynamics simulations. Three types of simulations are performed: with pre-existing vacancies in the dislocation core, with pre-existing interstitials, and without any pre-existing defects (intrinsic diffusion). We find that diffusion along the screw dislocation is dominated by the intrinsic mechanism, whereas diffusion along the edge dislocation is dominated by the vacancy mechanism. Diffusion along the screw dislocation is found to be significantly faster than diffusion along the edge dislocation, and both diffusivities are in reasonable agreement with experimental data. The intrinsic diffusion mechanism can be associated with the formation of dynamic Frenkel pairs, possibly activated by thermal jogs and/or kinks. The simulations show that at high temperatures the dislocation core becomes an effective source/sink of point defects and the effect of pre-existing defects on the core diffusivity diminishes.     

Modification of the jogged-screw model for creep of γ-TiAl

During the high-temperature creep of the γ-phase (L1₀ structure) of a “near-gamma” Ti–48Al microstructure, observations using transmission electron microscopy indicate that a/2〈110] or “unit” dislocation activity is a dominant deformation mode. These unit dislocations tend to be elongated along the screw orientation, and exhibit a large number of localized pinning points. Tilting experiments demonstrate that these pinning points are associated with jogs on the screw dislocations, suggesting that the jogged-screw model for creep should be appropriate in this case. However, it is shown that in its conventional formulation, the jogged-screw model is not capable of reproducing the measured creep response (i.e. stress exponents or absolute creep rates). Microscopic observations also demonstrate that a spectrum of jog heights are present, with some as large as 40 nm, based on present observations. A modification of the jogged-screw model is proposed in which the average jog height is assumed to depend on stress. This modified model results in good agreement between predicted and measured creep rates while using reasonable model parameters. Additional implications of the model and required experiments to further validate the model are also discussed.     

High temperature deformation of NiAl matrix composites

The intermetallic compound, NiAl, has many attractive properties as a high temperature structural material. However, its lack of creep resistance prevents practical applications. Adding ceramic reinforcements, such as TiB₂ particles, Al₂O₃ particles or whiskers can significantly improve the strength of binary NiAl at high temperatures. However, the increase in the yield stress of the discontinuous NiAl matrix composites as compared with monolithic NiAl is difficult to explain. The purposes of this research were to understand the deformation mechanisms which cause the increase in strength achieved by adding TiB₂ particles, Al₂O₃ particles or whiskers to NiAl, and to recognize the principles of the deformation process in NiAl matrix composites. In order to accomplish these objectives, mechanical properties and thermal activation parameters in NiAl matrix composites with different types, shapes and sizes of reinforcements have been systematically evaluated. Microstructures and dislocation structures in NiAl matrix composites have also been thoroughly characterized before and after deformation. It was found that the size of the reinforcement had a large influence on the microstructures of the composites, and the nominal activation energies for all the composites were the same and within the range of the activation energy of self-diffusion for pure NiAl. This indicated that the deformation mechanism was the same for all NiAl matrix composites. The thermally activated motion of jogged screw dislocations is postulated as the rate controlling mechanism of the deformation of NiAl matrix composites. However, this non-conservative motion of jogged screw dislocation theory requires only the appearance of vacancy producing (VP) jogs. A simple model which is based on the cross slip of screw dislocations in NiAl is proposed to account for the occurrence of VP jogs. A formulation of strain rate vs stress for the VP jogged screw dislocation model was derived. By computer simulation, it was found that this mechanism was capable of predicting the temperature dependence of the yield stresses of NiAl composites. It was further concluded that the reinforcement addition only increased the non-thermally activated component of the yield stress.     

Dislocation processes during the deformation of NiAl–0.2at.%Ta

Macroscopic compression tests and in situ straining experiments in a high-voltage electron microscope were performed on NiAl–0.2at.%Ta at room temperature and at elevated temperatures. At room temperature in soft orientations, dislocations of a〈100〉 Burgers vectors bow out between jogs. In contrast to pure NiAl, the dislocations move in a viscous way between the pinned configurations. At 475°C in a hard orientation, dislocations with a〈110〉 Burgers vectors move in a viscous way in configurations strongly depending on the respective slip plane. Preferred orientations of dislocations are of mixed character, most pronounced as very straight dislocations oriented along 〈111〉 directions on {110} planes. These configurations cannot be explained on the basis of the existing atomistic theories. The flow stress is interpreted in terms of the back stress of the dislocations bowing out between jogs at room temperature, the statistical theory of solid solution hardening, and the formation of atmospheres containing Ta atoms at elevated temperatures.     

A TEM in situ study of alloying effects in iron. II—Solid solution hardening caused by high concentrations of Si and Cr

The hardening effect of a high concentration of substitutional solute atoms in iron has been investigated by means of in situ straining experiments in FeSi and FeCr alloys, between 100 and 300 K. The results show that both screw and edge dislocations interact with solute atoms. This interaction is, however, strongest on screw dislocations, as a result of the formation of superjogs in the vicinity of solute atoms. Under such conditions, hardening takes place above a transition temperature for which the local pinning at superjogs becomes stronger than the Peierls friction stress.     

A dislocation dynamics analysis of the critical cross-slip annihilation distance and the cyclic saturation stress in fcc single crystals at different temperatures

Cross-slip is treated as a deterministic, mechanically activated process governed by the applied stress, by the interaction force between approaching screw dislocations of the opposite sign and by the line tension related to the persistent slip band channel width. The dislocations are modeled as moving polygons. In the evaluation of the critical cross-slip annihilation distance and the saturation stress in cycling, we accept two assumptions inspired by Brown [Brown L. Philos Mag A 2002;82:1691]: (i) the critical parameter associated with cross-slip is the spreading of the dislocation loop on the cross-slip plane, not the critical formation of a constriction in an extended screw dislocation core. (ii) The saturation stress in cycling is controlled by the stress required to separate two screw dislocations of opposite signs which are just on the point of mutual annihilation by cross-slip. The proposed model predicts the critical annihilation distance and the cyclic saturation stress in agreement with the available experimental data for Cu, Ni and Ag single crystals.     

Work hardening and recovery of gamma base titanium aluminides

Micromechanisms of deformation contributing to work hardening of γ-base titanium aluminides at room temperature have been investigated. Deformation has been considered as stress driven thermally activated process and the experiments described were designed to identify the nature of glide obstacles generated during deformation. The investigations involve mechanical testing, electron microscope observations of the defect structure and recovery experiments. The investigations give supporting evidence that work hardening is derived from long-range elastic interactions between dislocations on parallel and oblique slip planes. Another source of work hardening arises from dislocation dipoles and debris defects, which were trailed and terminated at jogs in screw dislocations and can be overcome with the aid of thermal activation. These defects probably give rise to a significant recovery of the work hardening upon annealing.     

Evaluation of the jogged-screw model of creep in equiaxed γ-TiAl: identification of the key substructural parameters

On the basis of microstructural observation of 1/2[1 1 0]-type jogged-screw dislocations, it has been previously proposed that the creep deformation mechanism in equiaxed γ-Ti–48Al alloys is controlled by the climb of the jogs on these dislocations. This is validated by predictions from a creep model based on these observations. However, several assumptions made in the model were not fully substantiated by experiment or theory. The aim of this study is to verify and validate the parameters and functional dependencies assumed in the model. In addition, the original solution has been reformulated to take into account the finite height of the moving jog. The substructural model parameters have been further investigated in light of this reformulation. The stress dependence of dislocation density, jog spacing and jog height has been evaluated via simulations, analytical modeling and experimental observations. Combining all of these parameters and dependencies in a reformulated model leads to an excellent prediction of creep rates and stress exponents.     

弹性交互作用下直刃型位错的稳态构型及其滑移稳定性

从各向同性连续介质中直线位错之间的弹性交互作用能出发，推导具有任意Ｂｕｒｇｅｒｓ矢量夹角的两平行直刃型位错之间弹性交互作用的滑移力攀移力表达式。指出两平行直刃型位错的弹性交互作用存在三个滑移平衡位置：ｕ１＝１，ｕ２＝－１，ｕ３＝ｔｇφ，并确定了三滑移平衡位置成为滑移稳定平衡位置的条件，还得到滑移，攀移共同运动情况下两平行直刃型位错在弹性交互作用下的稳态构型，分别讨论了紧束缚，弱束缚情况下位滑移的稳     

Relaxation of interphase stresses on the later stages of the heterogeneous decomposition of solid solutions: III. Conditions for the infiltration of feeding dislocations

In this paper, we have studied the relaxation processes that occur upon the decomposition of solid solutions at the stage of coalescence in the regime of dislocation-matrix diffusion using the precipitated-phase-particle-feeding-dislocations system as an example. The cases of linear and nonlinear interrelations between the controlling parameters of the system (the fraction of the relaxed regions of the interphase surface and the number of edge dislocations that supply the alloying component to the precipitated phase) have been analyzed. It has been established that in real cases it is advantageous for the system to reduce its total energy via the “infiltration” of feeding dislocations, i.e., the escape of segments of edge feeding dislocations localized in the precipitate outside the limits of the precipitate with the formation of structural dislocation loops at the interphase surface by the reaction of the following type: 1 feeding dislocation = 1 structural loop + 1 matrix dislocation. In the presence of an enhanced density of feeding dislocations, this reaction is blocked, and the relaxation of interphase stresses is accomplished as a result of sequential acts of the loss of coherence, which are accompanied by a partial “escape” of edge feeding dislocations. For the edge dislocations that remain unescaped, there is formulated a condition for the subsequent “leakage” of their segments localized in the precipitate outside the limits of the precipitate according to the following reaction: 2 feeding dislocations = 1 structural loop + 2 matrix dislocations.     

Temperature dependence of screw dislocation mobility on shuffle-set of silicon

Large-scale atomistic simulations are performed in order to observe local behaviors of screw dislocations located on the shuffle set of (111) in single crystal silicon, focusing on the propagation process of the screw dislocations. A quadrupolar arrangement of screw dislocations is utilized to impose the periodic boundary conditions along each of the three spatial directions. With the aid of molecular dynamics simulations, the dislocation mobility is investigated in terms of the critical resolved shear stress. Based on the results from the simulations, we discuss effects of the model size and temperature on the critical resolved shear stress. After choosing the proper model size to reduce undesirable interference between the dislocations, we further estimate the Peierls stress by fitting from a set of the critical resolved shear stresses at various temperatures. Meanwhile, we observe a double kink mechanism in the dislocation propagation which is the most energetically favorable dislocation movement in silicon. We investigate the formation and migration of kink pairs on an undissociated screw dislocation in silicon.     

Steady-State Creep Behavior of Super304H Austenitic Steel at Elevated Temperatures

Creep behavior of Super304 H austenitic steel has been investigated at elevated temperatures of 923-973 K and at applied stress of 190-210 MPa.The results show that the apparent stress exponent and activation energy in the creep deformation range from 16.2 to 27.4 and from 602.1 to 769.3 kJ/mol at different temperatures,respectively.These high values imply the presence of a threshold stress due to an interaction between the dislocations and Cu-rich precipitates during creep deformation.The creep mechanism is associated with the dislocation climbing governed by the matrix lattice diffusion.The origin of the threshold stress is mainly attributed to the coherency strain induced in the matrix by Cu-rich precipitates.The theoretically estimated threshold stresses from Cu-rich precipitates agree reasonably with the experimental results.     

Relaxation of interphase stresses on the later stages of the heterogeneous decomposition of solid solutions. II. Variational problem

The study of relaxation processes upon the decomposition of solid solutions at the stage of coalescence in the regime of dislocation-matrix diffusion is performed using a “precipitated-phase-particle-feeding-dislocations” system as an example. Within the framework of the variational approach, the cases of the independent and interdependent variation of the fraction of the relaxed regions of the interphase surface and of the number of edge dislocations which supply the alloying component to the precipitated phase have been investigated. Under the assumption that implies the linearity of the possible connection between these parameters, the model approximation of the continuous nucleation of epitaxial defects, and the absence of free matrix dislocations near the particle in the initial state, it is shown that the decrease in the number of edge feeding dislocations in the process of relaxation of interphase stresses can occur only by means of “leakage” of dislocation segments localized in the precipitate outside the limits of the precipitate with the formation of structural dislocation loops on the interphase surface.     

Atomistic aspects of the deformation of NiAl

Properties of dislocations in B2-NiAl have been studied atomistically using an embedded atom potential. The response of dislocation cores to applied homogeneous shear stresses is investigated and the Peierls stresses of straight dislocations are determined. The results are in many details in excellent agreement with experimental observations. Specifically, the behaviour of the 〈1 1 1〉 dislocations, their slip planes, cross-slip behaviour and the limiting role of the screw dislocation can be explained. Similarly, the appearance of the {2 1 0} plane as a secondary slip plane for the 〈1 0 0〉 dislocations can be rationalised. Furthermore, the interaction of the dislocations with structural point defects is studied. Comparison with the flow stress of off-stoichiometric NiAl from the literature shows that the individual point defects cannot be made responsible for the strong increase of the flow stress, suggesting that more complex defect structures may play an important role.     

Atomic core structure of 90°(c)-bent screw threading dislocations in wurtzite GaN

The atomic and electronic structures of the c threading dislocations with an edge or screw character were compared using a tight binding formalism which takes into account charge transfer.The two dislocations do not exhibit dangling bonds.While the screw dislocation contains only constrained Ga-N bonds, the edge dislocation contains Ga-Ga and N-N wrong bonds.Both dislocations are found to induce shallow and deep gap states.     

Oxygen–molybdenum interaction with dislocations in Nb-Mo single crystals at elevated temperatures

A study using a transmission electron microscope was performed for the deformation microstructures of Nb-20mol%Mo single crystals containing from 0.03 to 0.65 mol% oxygen and a Nb-40mol%Mo-0.05mol%O single crystal, compressed to approximately 4% plastic strain at 298 K, 973 K and 1473 K. At 298 K, screw dislocations are predominant in all alloys. At 973 K, in Nb-20Mo-0.03O and Nb-40Mo-0.05O, the dislocation microstructure contains the same amount of edge dislocations and small edge dipoles that tend to aggregate into clusters. In Nb-20Mo-0.65O, however, the arrangement consists of long glide loops parallel to the screw direction. At 1473 K, edge dislocations are dominant in all alloys. From the results, the following conclusions are made: (1) Oxygen atoms impede screw dislocation motion at 298 K and 973 K; (2) The influence of oxygen on screw segments is significantly strong at 973 K; (3) At 1473 K, oxygen atoms impede edge dislocation motion. It is inferred that O-Mo atmosphere is formed around dislocations.